It is known how to oxidize aromatic compounds possessing aliphatic substituents in the liquid phase using air or molecular oxygen. The addition of a catalyst was found to be effective for that type of oxidizing reaction. Generally the catalysts are selected from salts of metals having a variable valency such as cobalt, chromium, manganese, lead, iron, copper, nickel, vanadium, ruthenium, tungsten, cerium, molybdenum or mixtures thereof. While it is possible to produce aromatic monocarboxylic acids with relative ease, it is very difficult to produce aromatic dicarboxylic acids by one step oxidation of aromatic compounds having two oxidizable substituents.
Thus, in the known processes, the production of terephthalic acid from p-xylene is accomplished in two steps:
(a) p-xylene is reacted with oxygen in the presence of a cobalt or manganese salt, such as naphthenate, obtaining p-toluic acid, and PA1 (b) p-toluic acid is oxidized with nitric acid producing terephthalic acid. PA1 (a) a quaternary onium salt having the general formula: ##STR1## wherein: Y may be nitrogen or phosphorus, PA1 (b) a transition metal salt, the molar ratio between (a) and (b) being in the range of between 0.25:1 to 1.5:1. The above quaternary onium salts, possessing between 17 and 58 carbon atoms, are characterized by their lipophicity which enables the extraction of the transition metal salt. The most preferable phase-transfer catalysts are the quaternary onium salts having between 20 and 48 total carbon atoms.
However the use of a two-stage process is industrially disadvantageous. This is even quite complicated in view of the resistance of p-toluic acid to oxidation. In the past thirty years, it was discovered the unexpected effect of bromide ions on the catalytic oxidation of p-xylene in the presence of carboxylic acid as solvent. It was found that the activity of a soluble cobalt catalyst is enhanced by the presence of bromide ions, so that p-xylene in acetic acid is oxidised at high temperature producing terephthalic acid. Yields of 90-95% are mentioned to be obtained. Although the purity of the product was much higher than that obtained in the previous two-stage process, it was still not high enough for use in the manufacture of polyethylene terephthalate--one of the main uses of the terephthalic acid--by a direct esterification process. Therefore, an additional purification step was required in order to obtain the desired pure product.
The major organic impurities in the oxidation product of p-xylene are p-toluic acid and p-carboxybenzaldehyde. Both are mono-functional and therefore capable of limiting the rate of polymerization and the molecular weight during polyesters manufacture. Also, p-carboxybenzaldehyde particularly leads to discoloured polymer and can not be removed by a recrystallyzation process, because it is readily occluded in the terephthalic acid. Other impurities normally present, in relatively small amounts, are: brominated acids, benzyl dicarboxylic acid and fluorene dicarboxylic acid, which also affect the polymer colour.
A large number of patents appear in the last ten years describing various improvements in the process claiming to obtain terephthalic acid at high yields and of a high purity. Thus, according to the Russian Pat. No. 1171452 (cf. C.A. 104, 68602) the oxidation of p-xylene is carried out in acetic acid at 200-215 degrees C. at a pressure of 20-26 atmospheres in the presence of Co, Mn or Ni salts and bromide compounds. In a subsequent stage the reaction product is treated with steam at 180-200 degrees C.
According to Japanese Patent Application Kokai No. 83/189135 (Cf. C.A. 100, 86267) the oxidation is carried out in 3 stages, the reaction product from the second reactor being circulated with a pump and wet grounded. In this manner it is claimed to produce terephthalic acid with a low content of 4-carboxybenzaldehyde.
According to Czech Pat. No. 218,081 (C.A. 102, 221318) terephthalic acid is obtained in the absence of organic solvents by oxidation of p-xylene with a gas containing oxygen using a Br-activated transition metal salt and N-containing compounds or transition metal complexes, benzenecarboxylic acid and water. An example of catalyst given in this patent is Co-Br.sub.2 -pyridine complex.
In the Japanese Patent Application Kokai No. 84/190947 (Cf. C.A 102, 167,319) it is claimed the production of a high purity terephthalic acid from p-xylene and oxygen-containing gas in acetic acid with a very low content of 4-carboxybenzaldehyde. The process consists actually of two steps, wherein in the second step the mother liquor, after separating the formed terephthalic acid, is again oxidized in the presence of a new portion of catalysts. Another Japanese Kokai No. 84/190946 (Cf. C.A. 102,167,320) describes a variation of the above process, wherein the slurry without separating the resulted product is oxidized again in another reactor.
In a theoretical review which appears in React. Kinet. Catal. Lett. Vol. 27, No. 2, 231-3, there is a communication on the use of phase-transfer catalyst for the initiation of p-xylene oxidation with molecular oxygen in p-xylene-water system. It is mentioned that the maximum absorption rates of oxygen are influenced by the nature of the onium compound. Thus in the presence of cetyltrimethyl ammonium bromide, the maximum rate of p-xylene oxidation was 1.13 compared with 1.54 when pyridine was used. It is further stipulated that the onium compounds have a significant role during the initiation of chaiss. As the reaction proceeds, "peroxide compounds are rapidly produced and they interfere in initiation and propagation steps". Accordingly it is concluded that in the later stages of p-xylene oxidation the chain initiation is not the rate determining step "the action of the quaternary salts being masked with intermediately formed peroxidic species".
It is an object of the present invention to provide a simple process for the manufacture of dicarboxylic acids and derivatives thereof by oxidation of the corresponding aromatic hydrocarbons. It is another object of the present invention to provide a simple process for the manufacture of dicarboxylic acids at high conversion. It is yet another object of the present invention to provide a simple process for the manufacture of terephthalic acid and esters thereof.